Borussia Dortmund bus attacked, Marc Bartra injured

A letter found at the scene of a trio of explosions aimed at German football team Borussia Dortmund's bus has "claimed responsibility" for the attack, according to local police.

Police are refusing to divulge any of the contents of the letter until they can "check the authenticity intensively" - but they say the letter does not give any indication of the motive behind the explosions.

One more suspicious device has been located since police conducted a sweep of the area near the team's hotel with explosive dogs and drones.

The new developments come after centre back Marc Bartra was taken to hospital with a hand injury - and though initial reports suggested he was just in shock after suffering minor cuts, it has now been revealed that it's much more serious - so bad, in fact, that he requires surgery.

The team were en route to their stadium the Westfalenstadion to play their quarter-final clash against Monaco in the UEFA Champions League - but shortly after departing from the hotel to the stadium, there were three separate explosions near the team bus.

The bus is fitted with armoured window panes, which is thought to be the only thing that prevented fatal injuries to the players. Only one part of the bus - the back right window - was damaged badly by what have been described by police as "serious explosives".

The game has since been called off, and has been rescheduled for 6:45pm on Wednesday (7:45am Thursday NZ time), with all tickets purchased for Tuesday night's game valid or able to be refunded.

Stadium speaker Norbert Dickel told those in attendance to stay in the stadium despite the match cancellation, urging them to "keep calm in order to ensure an orderly departure".

Monaco fans have reportedly been chanting 'Dortmund' in solidarity with their opposition while they waited to leave.

Fans left the Westfalenstadion in a "peaceful and civilised" manner, according to police. It was said to have been a very quiet exit.

Dortmund's managing director Hans-Joachim Watzke says the entire club was left shocked by the incident.

"There was an explosive strike in the immediate vicinity of the hotel. Marc Bartra has been injured and treated by a doctor. He was caught in the hand," he said.

"It was immediately a very professional management of the crisis. I would like to give our fans, who have taken in the facts, a huge compliment. Our coach Thomas Tuchel was very shocked. One of the explosions went up directly on his side.

"The team is in shock. The pictures do not get you out of your head. For security reasons, we do not want to disclose anything about this procedure. The team will feel a very strong solidarity tomorrow."

Dortmund's players have now been taken back to their accommodation at the Mannschaftshotel l'Arrivée.